Following the move to electronic control, in which gasoline engines are controlled by ECUs (electronic control units), automobile engine ignition systems are now full-transistor systems, in which mechanical points (contact breakers) are not needed, and more recently, there has been a move to ignition systems that do not need distributors, in which the power distribution function is based on mechanical operations, or high-voltage leads (secondary leads) with which energy loss is unavoidable.
Direct ignition systems, which do not need distributors, are provided with a small independent ignition coil for each sparkplug, and ignition is performed by supplying low-voltage (equal to the battery voltage of 12 V) primary side current from an igniter unit. An ordinary connecting lead is sufficient to supply the primary side current to the small, independent ignition coils, and advances in ignition control units have made it possible to control the primary side current for all of the cylinders with a single igniter unit. As such, mechanical power distribution using a distributor is unnecessary.
Note that, in terms of publications relating to automobile engine ignition systems, the specification of Japanese Laid Open Patent Application JP-2004-239115-A discloses a battery voltage stabilization device for stabilizing battery voltage without lowering the same, even when the opening angle of the throttle is suddenly changed to full throttle. In other words, a voltage stabilization circuit for stabilizing battery voltage (12 V) having an electrolytic capacitor element is electrically connected between the positive terminal and the negative terminal of a battery for driving an engine by supplying current to sparkplugs, and a voltage equivalent to the voltage drop produced when there is a sudden variation in the load on the engine is compensated by current released from the electrolytic capacitor element of the voltage stabilization circuit so as to stabilize the battery voltage.
Furthermore, Japanese Utility Model JP-3106434-U proposes an automobile ignition stabilization device, which is a device connected directly in parallel to the automobile battery, as an ignition stabilization device capable of stabilizing the ignition period and producing a strong discharge, wherein an aluminum electrolytic capacitor having a capacitance of no less than 8000 μF, preferably 10,000 to 100,000 μF, and more preferably 15,000 to 60,000 μF, and an inspection device comprising an LED and a series resistors, are connected in series.
In engine ignition systems, whether these be older full-transistor ignition systems provided with a distributor, or direct ignition systems, as can be understood by the graph of measurement data for a primary coil side current waveform at the time of sparkplug ignition shown in FIG. 9, depending on the characteristics of the ignition coil, in the interval A immediately following sparkplug ignition (which is to say, immediately after breaking the primary side current), a negative back electromotive force (arrow C) is generated on the primary coil side. While the ignition energy is so large that the waveform in the arrow B interval is nearly vertical, this also disturbs the waveform.
Research by the present inventor has shown that the back electromotive force that occurs on the primary coil side influences the high-voltage generation on the secondary coil side and decreases the sparkplug voltage, which negatively impacts ignition efficiency. In particular, when the engine is operating at high speed, the secondary coil side sparkplug voltage is unstable and a time lag occurs, which is highly likely to cause ignition loss (misfiring).
The gist of the devices recited in both the aforementioned JP-2004-239115-A and JP-3106434-U is that an electrolytic capacitor having a large capacitance is connected in parallel directly between the positive and negative terminals of the battery, and stabilization is performed by compensating for sudden drops in battery voltage with current released from the electrolytic capacitor. While these devices stabilize the ignition system by way of stabilizing battery voltage, they do not assume countermeasures for the aforementioned problems of back electromotive force that occurs on the primary coil side of the ignition coils.
The present invention is a reflection of the situation described above, and an object thereof is to provide an auxiliary device for engine sparkplug ignition that allows for good sparkplug ignition in all speed ranges (and particularly at high speeds) by eliminating the back electromotive force that occurs in the primary side coil of the ignition coil in engine ignition systems modeled on existing automobiles.